Three kinds of DC polarization are tested. In two tests the current is the same, but in opposite directions. The third is a sham (placebo) condition with no current delivered. Each of the three scans is separated by at least 3 days. On each day, a series of scans is done in a single session. Before each injection of tracer, the DC current is turned on. This may cause a tingling or slight burning on the skin under the electrodes, which disappears when the current is turned off. In each session, the subject receives 16 injections of tracer about 8 minutes apart, with DC polarization turned on for 4 out of the 8 minutes.